With no mention of detrainable hexes in the supply sources in the Sicily game specific rules, does that mean that they are not a supply source in the game? With limited Rail Cap for the Allies and no Rail Cap for the Axis it made me wonder if the Detrainable hex source did not apply?

If the Game Specific Rules don't mention a rules change, then the Series Rules always apply.

From the Series Rules (4.2):

13.3c Detrainable Hexes. A railroadhex is detrainable if it meets all of theserequirements:• It has a village, city (any size), port,or Engineer-Capable HQ (13.8a).• It is under control (13.3h).• It has a rail path free of enemy ZOCback to a supply source. Extenderlinks can be part of this for tracesupply, but not for moving cargo.Note EZOC cannot be negated tomove cargo, but can to connect witha supply source.